Answer

2. Anakin Skywalker / Darth Vader - David Prowse / James Earl Jones (Parts IV-VI), Bob Anderson (Parts V-VI, lightsabre duels), Sebastian Shaw (Part VI finale), Jake Lloyd (Part I), Hayden Christensen (Parts II - III).
Fencing master Bob Anderson performed all the lightsaber duels in parts V & VI: in Part IV, David Prowse broke too many of the prop sabres. Sebastian Shaw played the unmasked Anakin Skywalker at the end of Part VI (although the 2004 DVD release replaces him with Hayden Christensen). This is supposedly the only time that a multi-movie character has been portrayed by three people in one film, let alone four (counting Jones as the voice).

3. Queen Elizabeth I - Bette Davis (The Virgin Queen, 1955), Annette Bening (Richard III, 1995), Judi Dench (Shakespeare in Love, 1998), Cate Blanchett (Elizabeth, 1998).
Dench won the 1998 "Best Supporting Actress" Oscar over Blanchett, the only time two people have been nominated for playing the same character.

4. Helen Keller, in various productions of "The Miracle Worker" - Patty Duke (1962), Melissa Gilbert (TV, 1979), Hallie Kate Eisenberg (TV, 2000). Patty Duke won an Oscar for her 1962 movie role, and then got the "Best Actress" Emmy over Melissa Gilbert (and 3 others) in 1979. Eisenberg was nominated for the Young Artist Award.